1. Field of the Invention
The present invention relates to a liquid crystal display device and more specifically to a liquid crystal display device with a high aperture ratio for achieving higher brightness, lower power consumption, and lower costs.
2. Related Background Art
A liquid crystal display device has a pair of transparent substrates placed opposite to each other. One is a thin film transistor array substrate which will be hereinafter referred to as a TFT substrate, and the other is a color filter substrate as a CF substrate. The substrates are provided with electrodes to drive a liquid crystal layer. On the TFT substrate, a switching element and a liquid crystal drive electrode and the like connected to the switching element are formed in an array arrangement. On the CF substrate, a black matrix (hereinafter referred to as a BM) arranged between color filters comprising of red, green, and blue films are formed corresponding to each switching element. Liquid crystal material is filled between the two substrates to display images by its polarization mechanism. A twisted nematic mode has been generally applied for driving liquid crystal, in which an electric field perpendicular to substrates is applied to liquid crystal to change the alignment direction of liquid crystal molecules from horizontal to vertical.
The twisted nematic mode, however, has the problem of a narrow viewing angle since the liquid crystal molecules aligned vertically by the electric field make a certain angle with the substrates to vary brightness depending on the direction
As a solution to the above problem, a liquid crystal display device of an In-Plane Switching mode (which will be hereinafter referred to as an IPS mode”) has come into use, in which an electric field substantially parallel to substrates is applied to liquid crystal to cause liquid crystal molecules to rotate in the horizontal direction.
In the IPS mode liquid crystal display device, liquid crystal molecules are always parallel to substrates, and therefore a viewing angle is significantly wider than the twisted nematic liquid crystal display device. The IPS liquid crystal display device has come to the front for use as a monitor.
In a liquid crystal panel, an adhesive agent called sealing material is applied to the periphery of the CF substrate being adhered to the TFT substrate. There is a gap between the two substrates, in which liquid crystal material is filled. It is difficult to have the uniformity of the gap over an entire substrate, and therefore the gap varies between places, causing brightness and color to change to produce uneven picture. Since the change in brightness and color according to the gap is generally greater in the IPS mode than in the twisted nematic mode, the IPS mode especially requires the uniformity of the gap over the entire substrate.
In order to ensure the uniformity of the gap width, a number of spacers are generally provided over the plane between the substrates. The spacers are generally formed on the CF substrate in the configuration shown in FIG. 9. The reference numeral 8 designates the spacer, 9 a BM as a light-shielding layer, 10 a color filter as an RGB pixel, and 13 an overcoat layer to planarize the surface of the CF substrate.
Conventionally, when the CF substrate and the TFT substrate are overlapped, the spacer 8 is located in the position shown in FIGS. 10 and 11 on the TFT substrate. The reference numeral 1 designates a gate line, 2 a source line, 3 a source electrode, and 4 a drain electrode, which constitute a switching element of a TFT. The reference numeral 5 designates a liquid crystal drive electrode connected to the drain electrode 4, 6 a common electrode placed opposite to the liquid crystal drive electrode 5, and 7 a common capacitor line connected to the common electrode 6. An electric field between the liquid crystal drive electrode 5 and the common electrode 6 causes liquid crystal material to align. In the IPS mode, the liquid crystal drive electrode 5 and the common electrode 6 are comb-shaped to apply a charge horizontally. The BM 9 are placed in the position shown by a heavy line in FIGS. 10 and 11 (the position corresponding to the gate line 1 and the source line 2) on the CF substrate in order to block light between pixels. FIGS. 10 and 11 are enlarged views of one pixel, and the pixels are arranged in an array arrangement.
In the arrangement shown in FIG. 10, the spacer 8 contacts the TFT substrate on the source line 3. The normal diameter of the spacer 8 is approximately 10 to 20 μm. Since the pixels are arranged in an array arrangement, the line width e of the BM 9 is the sum of c plus d in FIG. 10 which is approximately 15 μm being substantially equal to the spacer 8.
The height of the spacer 8 should be equal in order to have the uniform width of a cell gap (the gap between the two substrates). Therefore, when the substrates are arranged facing each other, the spacer 8 should not overlap with the color filters 10 nor stick out the BM 9. Also, the BM 9 and the color filters 10 should overlap in order to avoid decoloring to increase a panel display quality. However, with the width accuracy and the position accuracy with respect to the BM 9 of the spacer 8, the width accuracy of the BM 9, and the width accuracy and the position accuracy with respect to BM 9 of the color filters 10, the BM 9 should have a width of approximately 10 to 15 μm each side, causing low aperture ratio and low brightness. There is thus the problem that a less costly backlight with lower power consumption is unachievable in a liquid crystal display device.
In the arrangement shown in FIG. 11, the spacer contacts the TFT substrate on the gate line 1. The line width e of the BM 9 in this arrangement is the sum of a plus b in FIG. 11 which is approximately 30 μm.
However, in order to provide the spacer 8 avoiding an uneven gap, the width of the BM should be wider (approximately 35 to 45 μm) in this case as well. Accordingly, an aperture ratio is decreased to reduce brightness. The conventional liquid crystal display device thereby has the problem that a less costly backlight with lower power consumption is unachievable.
Further, in the conventional TFT substrate shown in FIGS. 10 and 11, even when the widths of the gate line 1, source line 2, and BM 9 are narrowed down for a higher aperture ratio, it is impossible to narrow the width of the BM around the spacer 8, thus hindering increase in the aperture ratio.